The present invention relates to a laser-assisted friction stir welding method for joining workpieces.
The basic principles of friction stir welding (FSW) are known, for example, from European Patent EP 0 615 480. Two workpieces to be joined are brought into contact along a joint area, held and secured in this position. A probe of a harder material than the workpiece is plunged into the joint area and into the workpiece material on both sides of the joint area while being in rotary motion. In the process, the probe generates frictional heat. Thereupon, the opposing workpiece regions along the joint line assume a plasticized state. The probe is moved forward along the joint line so that the material of opposing workpiece regions which is located in front of the probe is plasticized and the plasticized regions behind the probe solidify. The probe is removed from the joint area before the material completely solidifies. Materials such as metals, alloys thereof, metal composite materials (so-called xe2x80x9cMCCxe2x80x9d) or suitable plastic materials are welded together in this manner.
A further, improved friction stir welding method whereby a smaller number of defects and a smooth surface of the processed material are achieved is known, for example, from European Patent EP 0 752 926. This publication describes a modified probe arrangement. The rotating probe which is plunged into the joint area is tilted with respect to the vertical so that the probe points in the direction of forward movement. Because of this, the plasticized material created in the joint area is exposed to a perpendicular pressure along the surface of the workpieces. This leads to improved material flow, thus resulting in a more homogeneous weld. In this manner, it is possible to make joints having a smaller number of defects and a smooth surface.
Another friction stir welding method is known, for example, from World Patent WO 99/39861, which describes a method in which an additional heat source is used. The heat source is used to heat the region immediately in front of the rotating probe. This results in a more effective plasticization of the material because not only the frictional heat of the rotating probe is used but also the additional heating of the separate heat source. Additional heat sources that can be used include, for example, resistance heaters, induction coils, high-frequency induction coils or lasers.
The use of resistance heaters is disadvantageous because of the relatively high electric currents flowing between the tool and the workpiece. Even with good protection, an impairment of the environment, in particular of people, cannot be ruled out. Furthermore, electrically conductive materials for the workpiece and the tool are a prerequisite. Consequently, it is not possible for the FSW tool to be desirably formed of coated, metallic or ceramic materials.
In addition, the known methods have the disadvantage that often only a limited process speed is permitted, depending on the Al alloy to be joined. This is problematic, in particular, when processing thick workpieces (typically greater than 6 mm). In the case of thick workpieces, there is also the problem of asymmetric or uneven heat conduction within the workpiece material so that the known methods are unsuitable for processing workpieces of that kind.
An object of the present invention is to provide a method for friction stir welding which allows short welding times and an excellent joint quality, in particular in the case of thick workpieces.
The present invention provides a laser-assisted friction stir welding method for joining workpieces, that includes the step of providing a side face for each workpiece between oppositely facing first and second workpiece surfaces, each side face shaped such that, in a pressed-together condition, the side faces touch each other at the second workpiece surface and are spaced from each other in a middle region of the side faces and so that a gap exists between the side faces at the first workpiece surface. The method also includes pressing together the side faces so as to form a joint area, and advancing a welding probe, while the probe is in rotary motion, along the joint area at the first workpiece surface and irradiating workpiece material located in front of the welding probe using laser radiation so as to plasticize the workpiece material along the joint area. The method also includes removing the welding probe from the joint area before the material completely solidifies.
In this context, the abutting faces of the workpieces to be joined together have a special shape so that the energy that is additionally radiated from a laser is effectively used for the heating of the material to be plasticized. To this end, the workpiece sides to be joined are designed in such a maimer that, in the pressed-together condition, the side faces touch each other in the root region of the joint profile, that a gap is present between the pressed-together side faces at the workpiece surface facing toward the tool and the laser, and that the joint profile has a clearance or hollow space in the middle region. In this manner, the unwanted back-reflection of the laser radiation at the workpiece surface is markedly reduced or completely avoided. Moreover, due to the adapted side faces it is achieved that the laser energy centrally enters the middle region of the workpieces and from there is uniformly distributed over the entire cross-section because of heat conduction. This allows a much more efficient heating of the workpiece material and is advantageous, in particular when processing specimens that are thicker than 6 mm.
According to a first embodiment of the present invention, the to-be-joined side faces of the workpieces are beveled in such a manner that the workpieces touch each other in the root region of the joint profile and a gap is present between the side faces at the workpiece surface facing toward the tool and the laser, i.e., the workpiece surface opposite the root region.
According to a second embodiment of the present invention, the to-be-joined side faces of the workpieces are beveled in the shape of a parabola or an arc and touch each other in the root region of the joint profile. Starting at the root region, the side faces are spaced from each other by the bevel, the clearance increasing in the direction of the workpiece surface facing toward the tool and the laser.
According to a further embodiment, the to-be-joined side faces of the workpieces have a semicircular or arched profile as viewed in the cross-section of the joint so that, in the pressed-together condition, the workpieces touch each other in the root region and a gap is present between the side faces at the workpiece surface facing toward the laser. In this case, the clearance between the side faces initially increases, starting at the root region, and then decreases toward the workpiece surface from the maximum in the middle region to the value of the gap width.
According to a further embodiment, the to-be-joined side faces of the workpieces are formed in such a manner that, in the pressed-together condition, the joint profile of the side faces has the shape of a so-called xe2x80x9cbeam trapxe2x80x9d (xe2x80x9cUlbrich spherexe2x80x9d).
Using the method according to the present invention, the regions to be plasticized are heated in a more effective and more uniform manner, as a result of which faster joining speeds and better joint quality are achieved. Due to the higher local workpiece and process temperature, on one hand, the process forces decrease because of the better plasticization and, on the other hand, the joining speed is increased. This is an advantage especially for joining thick cross-sections. Thus, the application area for the FSW method is increased.
It is also beneficial that the method according to the present invention offers more processing reliability and, moreover, involves a reduction in load for the FSW process machines. In the long term, therefore, it is also possible to save costs in production.